$out ,$projection in mongodb - node.js

Question

i want to save records in a new collection using either $out or $merge.
**/////collection 2- reservationdeatils////**
"_id":ObjectId("5e4a898947363e964a886420"),
"phoneNo" : 98765#####,
"name" : "name1",
"userId":ObjectId("5e1efac668c3c811c83263cc"),
"approversId":ObjectId("5e1efad268c3c811c83263cd")
"bookedForDate":ISODate("2020-02-20T07:23:36.130Z"),
"bookingDetails" : [
{ "_id" : ObjectId("5e44f471d1868d2a54aac12d"),
"seatsBooked" : 15,
"floorId" : "#IKE01",
},
{ "_id" : ObjectId("5e44f471d1868d2a54aac12c"),
"seatsBooked" : 35,
"floorId" : "#HKE04",
}
],
**/////collection 2-priceDetails////**
{
"_id" : ObjectId("5e1efb0168c3c811c83263ce"),
"floorId" : "#IKE01",
"weekday" : "monday",
"pricePoint" : 589,
}
{
"_id" : ObjectId("5e2694db54e532a4eb92b477"),
"floorId" : "#IKE02",
"weekday" : "thursday",
"pricePoint" : 699
}
{
"_id" : ObjectId("5e2694f954e532a4eb92b478"),
"floorId" : "#HKE04",
"weekday" : "monday",
"pricePoint" : 579
}
**/////collection 3- discount////**
{
"_id" : ObjectId("5e427de64617181a4ce38893"),
"userId" : ObjectId("5e3d05ba964d0e06c4bb0f07"),
"approversId" : ObjectId("5e1d82156a67173cb877f67d"),
"floorId" : "#IKE01",
"weekday" : "monday",
"discount" : 20%,
},
{
"_id" : ObjectId("5e4281e7fec2e01a4c60b406"),
"userId" : ObjectId("5e1efac668c3c811c83263cc"),
"approversId" : ObjectId("5e1efad268c3c811c83263cd"),
"floorId" : "#IKE01",
"weekday" : "monday",
"discount" : 24%,
}
Now below is the query i have tried :
db.reservationdeatils.aggregate([
{
'$match': {
'approverId': ObjectId('5e1efad268c3c811c83263cd'),
'userId': ObjectId('5e1efac668c3c811c83263cc'),
'bookedForDate': ISODate("2020-02-11T18:30:00Z"),
}
},
{
'$unwind': {
'path': '$bookingDetails',
},
},
{
$lookup:
{
from: 'priceDetails',
let: { floorId: '$bookingDetails.floorId' },
pipeline: [
{
$match: {
weekday: 'monday',
$expr: {
$eq: ["$floorId", "$$floorId"]
}
}
}
], as: 'priceDetails'
}
},
{ '$unwind': '$priceDetails' },
{
$lookup:
{
from: 'discount',
let: { floorId: '$bookingDetails.floorId' },
pipeline: [
{
$match: {
weekday: 'monday',
$expr: {
$eq: ["$floorId", "$$floorId"]
}
}
}
], as: 'discounts'
}
},
{ '$unwind': '$discounts' },
{
'$group': {
'_id': {
'floorId': '$bookingDetails.floorId',
'date': '$bookedForDate',
'price': '$priceDetails.pricePoint',
'discount': '$discounts.discount'
},
'seatsBooked': {
'$sum': '$bookingDetails.seatsBooked'
},
}
},
{
'$project': {
'amount': {
'$multiply':
[
'$seatsBooked',
{'$subtract':
['$_id.pricePoint',
{ '$multiply':
['$_id.pricePoint',
{ '$divide':
['$_id.discount', 100]
}]
}]
}]
},
},
},
{
$group: {
_id: null,
totalAmount: {
$sum: "$amount"
}
}
},
{
'$project': {
_id:0,
totalAmount:1,
bookedForDate:1,
'floorId':'$priceDetails.floorId'
}
},{'$merge':'invoice'}
]).pretty()
i have been able to achieve the totalAmount but what i want to achieve is that i want to save these fields into "invoice" collection "userId","approversId","floorId","sum","totalSum","bookedForDate","name" BUT 1:whenever i use $out instead of $merge the previous document gets replaces which i dont want, 2: if i use $merge everytime i run the query a new document is created and that too only with _id:ObjectId(5e4a899c47363e964a88642f),totalBill:#### these fields , any suggestion how can i achieve this

Answer 1

You are going in a good direction you just need to look at the $group aggregation.
One more thing I have used the discount as int value, not in percentage.
"discount" : 24
I have updated the query:
db.reservationdeatils.aggregate([
{
$match: {
"userId" : ObjectId("5e1efac668c3c811c83263cc"),
"approversId" : ObjectId("5e1efad268c3c811c83263cd"),
"bookedForDate" : ISODate("2020-02-20T07:23:36.130Z")
}
},
{
$unwind: {
path: "$bookingDetails",
},
},
{
$lookup:
{
from: "priceDetails",
let: { floorId: "$bookingDetails.floorId" },
pipeline: [
{
$match: {
weekday: "monday",
$expr: {
$eq: ["$floorId", "$$floorId"]
}
}
}
],
as: "priceDetails"
}
},
{
$unwind: "$priceDetails"
},
{
$lookup:
{
from: "discount",
let: { floorId: "$bookingDetails.floorId" },
pipeline: [
{
$match: {
weekday: "monday",
$expr: {
$eq: ["$floorId", "$$floorId"]
}
}
}
],
as: "discounts"
}
}
,
{
$unwind: "$discounts"
}
,
{
$group: {
"_id": {
"floorId": "$bookingDetails.floorId",
"date": "$bookedForDate",
"price": "$priceDetails.pricePoint",
"discount": "$discounts.discount"
},
"price":{
$first:"$priceDetails.pricePoint"
},
"discount":{
$first:"$discounts.discount"
},
"seatsBooked": {
$sum: "$bookingDetails.seatsBooked"
},
}
}
,
{
$project: {
"amount": {
$multiply:
[
"$seatsBooked",
{
$subtract:[
"$price",
{
$multiply:[
"$price",
{
$divide:[
"$discount",
100
]
}
]
}
]
}
]
},
},
}
,
{
$group: {
"_id": null,
"totalAmount": {
$sum: "$amount"
}
}
},
{
$project: {
"_id":0,
"totalAmount":1,
"bookedForDate":1,
"floorId":"$priceDetails.floorId"
}
},
{
$out:"invoice"
}
]).pretty()
This will help you.